The River Mersey is a river in north west England. It is 70 miles (113 km) long, stretching from Stockport, Greater Manchester, and ending at Liverpool Bay, Merseyside.

For centuries, it formed part of the ancient county divide between Lancashire and Cheshire.

The Mersey is formed from three tributaries: the River Etherow, the River Goyt and the River Tame. The modern accepted start of the Mersey is at the confluence of the Tame and Goyt, in central Stockport, Greater Manchester. However, older definitions, and many older maps, place its start a few miles up the Goyt; for example the 1911 Encyclopedia Britannica states "It is formed by the junction of the Goyt and the Etherow a short distance below Marple in Cheshire on the first-named stream."

From Stockport it flows near East Didsbury, Stretford, Urmston and Flixton, then at Irlam it flows into the Manchester Ship Canal, which canalised the River Irwell to this point. The course of the Mersey has been obliterated by the Canal past Hollins Green to Rixton although the old river bed can be seen outside Irlam and also at Warburton; at Rixton the River Bollin enters the Canal from the south and the Mersey leaves the Canal to the north, meandering through Woolston, where the Ship Canal Company's dredgings have formed a nature reserve (Woolston Eyes), and Warrington.

It is tidal from Howley Weir in Warrington, although high spring tides often top the weir. A small bypass around Howley Weir, Howley Lock, was created before the ship canal existed, but is now redundant. The lock can still be seen to this day.

West of Warrington the river widens, passing through the Runcorn Gap between the towns of Runcorn and Widnes, in Halton. The Manchester Ship Canal also lies in the Gap, along the southern bank of the river. The Runcorn Gap is bridged by the Silver Jubilee Bridge and Runcorn Railway Bridge, while a project known as Mersey Gateway to build a new road bridge over the Mersey east of the existing bridges is currently under consideration, and has received some government support.

From the Runcorn Gap, the river widens into a large estuary, which is three miles (5 km) wide at its widest point near Ellesmere Port. The course of the river then heads north, with Liverpool to the east and the Wirral Peninsula to the west. The Manchester Ship Canal continues along the Cheshire bank of the river as far as Eastham Locks, where it enters the river.

The eastern part of this estuary is much affected by silting, and part of it is marked on modern maps as dry land instead of as tidal. These wetlands are of importance to wildlife, and are listed as a Ramsar site.

The estuary then narrows to flow between Liverpool and Birkenhead, where it is constricted to a width of 1000yds, between Albert Dock and the Woodside ferry terminal.

It then flows into Liverpool Bay on the Irish Sea, after a total course of around 70 miles (110 km). The conurbation on both sides of the river in this area is known as Merseyside.

Two road tunnels run under the Mersey at Liverpool: the older Queensway Tunnel (opened 1934) connecting with Birkenhead, and the Kingsway Tunnel (opened 1971) connecting with Wallasey. There is also a railway tunnel dating back to the 1880s, which carries passenger services on the Wirral Line of the Merseyrail franchise.

The Mersey Ferry runs between the Pier Head at Liverpool, and the Wirral terminals at Seacombe, Wallasey and Woodside, Birkenhead.

One explanation for the river's name comes from Anglo-Saxon M?res-ea = "border river", likely because it was the border between Mercia and Northumbria. Another explanation is possible: Mære can also mean "lake, pond, mere, water basin, sea". The old Welsh name could be môr-afon (sea-river) or môr-dwfr (sea-water). Mære , môr and Latin mare are an old Indo-European word. It is possible that the Anglo-Saxons simply anglicized môrafon. Given the fact that the estuary resembles a mere, this interpretation is rational. The argument against "border river" as an original name, is that the river would have obtained its name after it became a border. In the old days, rivers were rarely borders, as they provided a mean of transport. The same people would settle on both river banks. In addition, no river is known to have obtained its name according to a political event. The author who wrote the river name for the first time, must have noticed the strong similarity with border, as the river had become a border at that moment. The word Meuse was also a border river between the Romans and the Germans. Or Perhaps we should go back to Egypt, Moses and Musa 'Mouss' the addition of an 'R' just means a long distance. So that a Mose is a Scandinavian lake, wheras Marsh is a strech of land along a body of water.

Water quality in the River Mersey has been severely affected by industrialisation in the region, and in 1985, the Mersey Basin Campaign was established to improve water quality and encourage waterside regeneration.

In 2002, oxygen levels that could support fish along the entire length were witnessed for the first time. Salmon are now found in the river. They can be viewed on the Salmon Steps at Woolston between the months of September and November.

The river is now internationally famous thanks to the music of the 1960s known as Merseybeat and its strong association with Liverpool. The Mersey itself was popularized in the Merseybeat song Ferry Cross the Mersey by Gerry & The Pacemakers. The group later recorded a follow-up, Mersey Lullaby, that is part of the 2007 childrens' CD/book Blue Moo: Jukebox Hits from Way Back Never, by Sandra Boynton. Also, Paul McCartney's 2007 song That Was Me, from his album Memory Almost Full mentions merseybeating with the band and the song 'Mersey Paradise' by the stone roses has the song in the title.

The Mersey is considered sacred by British Hindus, and is even worshipped as equivalent to the River Ganges. After a ceremony on the river in September 2007, plans are underway for a large-scale event in 2008 (the year Liverpool held the title of European Capital of Culture).